Flux gate transducer



Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Sept. 5, 1961 D. E. WIEGAND 2,999,135

FLUX GATE TRANSDUCER Filed March 5. 1955 5 Sheets-Sheet 2 /001 /a/KL /045 Sheets-Sheet 3 IIIIIIIIIIIIIIII Sept. 5, 1961 Filed March 3. 1955 .ZhVE n fr" .PAW/ f h//fwv 4 a a f g\ a I. z M

/94 W Ma-m, @M- l Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Filed March 5. 1955 5 Sheets-Sheet 4 Erg. /Z

Sept. 5, 1961 D. E. wlEGAND 2,999,135

FLUX GATE TRANSDUCER Uur-Pur 5.

United States Patent 12,999,155 g g FLUX GATE TRANSDUCER -David E.Wiegand, Villa Pai-k, Ill., assignor to Armour 'Research Foundation ofIllinois .Institute of Technology, Chicago, Ill., n corporation ofIllinois Filed Mar. 3, 1955, Ser. No. 492,013 4 Claims. (Cl. 179-1001)The present invention is concerned with an improved 'electromagnetictransducing head Tand method, and is'par- `ticularly directed to an'electromagnetic reproducing head capable of response directly "tomagnetic flux rather'than to rate of change of llux.

An object of the'present invention is to provide an improvedma'gnetictransducin'g assembly which is sensitive directly to magnetic 'fluxrathefthan'to 'the rate of change thereof.

Another object of thepresent 'invention'is to provide an improvedmagnetic reproducing head capable of detecting 4recorded signals ofvariable frequency, .including zero fre- 'quency Another object of thepresent invention is to provide an improved magnetic reproducing head,and method pro- "vid-ing va substantially vhigher signal output thanconventional magnetic recording "heads A'further object ofthepresent'invention'is to provide 'a .novel magnetic modulator head andmethod usable for *the reproduction of music tand 'other signalsrequiring a A still further object of the :present invention is toprovide a magnetic modulator head of novel and improved construction.

The novel Vfeatureswhich f-I 'b'elieve 1 to be characteristic ofmy'invention are 'set forth with particularity in the appended claims.My invention itself, :'however, as to its organization, manner ofconstruction'and 'method of operation, .together with'further objectsand advantages thereof may be best understood by reference to thefollowing description, taken in connection with 'the accompanyingdrawings, in which:

FIGURE l is a somewhat 'diagrammatic top plan view of Va firstelectromagnetic transducer head according to the present'invention;

FIGURE 2 isa longitudinal sectional view of the head of FIGURE 1;

FIGURE 3 is 'an end elevational view of the head of FIGURE 1;

FIGURE 4 is a diagrammatic view illustrating the energizingand outputcircuits for the head of FIGURE l;

'FIGURE 5 is va'diagrammatic lsectional 'view of a second form ofelectromagnetic transducer head according to the present invention;

FIGURE Sais a diagrammatic-cross sectional view Ataken along the lineVa-Va'of FIGURES;

FIGURE 5b is a diagrammatic view similar to FIG- URE 5a, but showing amodified energizing circuit.

.FIGURE 6 is a diagrammaticview illustrating a third form of headconstruction;

FIGURE 7 is a diagrammatic side elevationalview of a further headconstruction Aaccording to 'the present in- `vention;

FIGURE 8 is a top plan View 'of the head of FIG- 'URE 7;

FIGURE 9 Yis 'a diagrammatic side elevational View of 'a vstill further'head constructed in accordance with the principles of the presentinvention;

FIGURE 10 is a diagrammatic view illustrating the energizing and outputcircuits for the head of FIGURE 9;

FIGURE l1 is a diagrammatic side elevational view of la further headconstruction;

FIGURE 12 is 1a diagrammatic Side elevational view of a still furthermodification;

rice

FIGURE 13 is a diagrammatic side elevational'fview of a further headconstruction utilizing a unidirectional high frequency winding on asaturating strip;

FIGURE 14 illustrates a modification utilizing fhigh frequency windingsconnected in aiding relation with-frespect to the gap and operating onthe incremental permeability characteristics of the record member;

FIGURE 15 illustrates a magnetic head excited hya high frequency pulsegenerator;

FIGURES 16 and l7 illustrate -a manner vof applyi'gl winding to astaturating strip without wrappinglthe winding completely around thesaturating strip;

FIGURES 18 and 19 illustrate a further manner :fdr exciting a saturatingstrip withoutthe use of a completely encircling the strip;

FIGURE 2O represents graphically the signal'and "no'ise output from ahead according to the present linvention as a function of the appliedintensity of magnetization from the oscillator; and

FIGURE 21 is a diagrammatic illustration of ama'gnetic head havinglaterally offset poles.

As shown on the drawings:

Referring to FIGURES 1 through 4, there is illustrated a shieldedtransducer head construction according 'to present invention.

The head comprises a pair of pole shoes '10 and t1 which are illustratedas being of the confronting type-5to define therebetween a non-magneticgap 12. Th'e A"ple pieces are shaped to provide tape contacting surfacesfor receiving a tape magnetic record member 14 in proxirrty to thenon-magnetic gap 12. The pole shoes'dU-and ll are secured in anysuitable manner within a housing whdh includes a pair of Mumetal shieldplates 16 and 17, n'o'nmagnetic spacers 19 and 20 insulating the pole`shoes the shield plates. The shield plates 16 and x17 lmay be providedfrom an integral strip of magnetic mateil formed into a generallyU-shape with abase :portion 21 of the U being formed with upstandinglugs '22 and 2B, the lugs being formed from the material of slotsY2'4f`atld 25 in the base 21.

Windings 29 and 30 are mounted on saturatng'mernbers 31 and 32 whichbridge between the pole shoes .1'0 and 11 and the lugs 22 and 23 tocomplete a single loop magnetic circuit including the base portion 211`and Cthe gap 12.

Referring to FIGURE 4, winding portions 29a and 30a of windings 29 and30, respectively, are connected in 'series opposing relation betweenleads 40 and 41 from asuitb'le oscillator 42 which is capable ofgenerating a signal fof frequency substantially above the frequencies tobe reproduced from the magnetic impulse record member 14, the output ofthe high frequency source being, for example, on the order of 228kilocycles per second. While there are a number of suitable ways forobtaining a signal from the head construction of FIGURES 1 4, FIGURE A4illustrates winding portions 29b and 30b of windings 29 30,respectively, providing pickup windings connected in series aidingrelation with respect 'to output leads '43 44. The output signal fromleads 43 and 44 may delivered through a condenser 45, an amplifier 46,`and a detector 47 to an output device such as loudspeaker "48. The highfrequency excitation windings 29a and 30g are preferably connected inseries opposing relation with spect to the magnetic circuit asillustrated so asf'to -a'vid the generation of a net high frequency leldat the "n'o'nmagnetic gap.

A D.C. polarizing flux maybe introduced into the-inagnetic circuit bymeans of resistor 49 and batteryS Lrto unbalance the high frequency uxesin the magnetic cuit. With such a polarizing ux, the -polarity 'o'f thejiltput from the detector 47 will vary 'in-accordance recorded signal.The level of D.C. polarizing -'x is .ux density in the saturating rangebetween l/a and -medium plus the polarizing flux equals 1/a to 3.preferably higher than the maximum signal flux in the saturating stripsto avoid distortion. In the absence of a D.C. bias liux, the outputsignal can be passed through a Iphase demodulator or phase sensitiverectifier so as to obtain an output the amplitude of which varies inaccordance with the polarity of the recorded signal. The phaseldemodulator may comprise a full wave rectifier network with theamplified output from the pickup windings 29h, 30b transformer coupledto one pair of terminals and a double frequency reference signaltransformer coupled to the other pair of terminals of the rectifiernetwork, the output being taken between center taps on the secondaiies-of the two transformers. 'I'he rectifier network may comprise fourrectifiers connected in aiding relation abouta closed circuit with thetransformer secondaiies each connected between one pair of oppositeterminals of the network. A suitable phase demodulator is shown in mycopending application Serial No. 294,684 of which the presentapplication is a continuation-impart.

`As illustrated by the arrows 51 and 52 in FIGURE 4, the excitationwindings 29a and 30a produce oppositely directed fluxes in the magneticcircuit including the saturating strips 31, 32 and pole pieces 10 andy11 so that the high frequency fluxes may balance out at the gap 12. Asdiagrammatically indicated in FIGURE 4, the high frequency ux paths 51:and 52 may include portions of the l. magnetic shield members, hereindicated as a single flat plate 55 with a rectangular central s'lot at56. In the construction of FIGURES l-3, it will be apparent that theleakage high frequency tlux paths for each winding includes portions ofeach of the shield plates 16 and 17.

It has been discovered that the cross section of the saturating strips31 and 32 is critical to a satisfactory signalto-noise ratio for certainapplications, and that such a satisfactory signal-to-noise ratio isobtained by substantially reducing the cross-sectional area of thesaturating parts. Further, it has been found that, within limits, thestrength of the useful signal from the head is essentially unaffected bythis reduction in cross-section.

More specifically, I have found that the optimum signalto-noise ratioand linear operation are obtained when the saturating parts 31 and 32are narrowed down so that a maximum signal on the tape 14 will produce amembers 31 and 32, which when added to any polarizing ux density iswithin the 1% of the value of intrinsic saturation induction for thematerial of the saturating members. In other words, the maximum fluxcapacity of the record of the product of the value of the saturationinduction for the material of the saturating members and the crosssection thereof.

By way of example, a specic head construction according to theembodiment of FIGURES 1-4 may comprise a pair of .006 inch thick by .060inch wide Mumetal pole shoes, .010 inch thick insulating spacers, .020inch thick Mumetal shield plates, and saturator strips of .001 inchthick by .010 inch wide by .380 inch long Molypermalloy. Mumetal maycomprise an alloy of copper, 2% chromium, 77% nickel, and the remainderiron Iand minor constituents, while Molypermalloy or MolybdenumPermalloy may comprise 4% molybdenum, 79% nickel and the remainder ironand minor constituents.

FIGURES 5 and 5a illustrate a head construction including a magne 'ccore portion formed from a magnetic lamination 80 of block Iconfiguration folded about a conductor strip 81 to provide enlargedcross-section pole portions 80a and 80h having upper edges 80e and 80dfor receiving -a tape magnetic record member successively thereacross.The connecting portion of the core 80e (the vertical part of the I) isof reduced cross-section such that a maximum signal on 'the tape willproduce a tlux density in the saturating portion 80e which when added tothe polarizing ux density therein produced by magnet 81' of the order ofB/s the value of saturation induction for the material of the lamination80.

The lamination may have an exciting winding 83 thereon connected with anoscillator 84 for establishing bucking magnetic fields at the gapbetween poles 80a and 80b. FIGURE 5b illustrates a modification whereina high frequency conductor 83a links the magnetic circuit provided bylamination 80 to establish a circulating high frequency ux in thelamination and across the gap between poles 80a and 80b. The value ofcirculating ux is selected to be too low to erase the signal on therecord member 85. Such a circulating iiux will still provide the desiredmagnetic modulator action. As indicated, the exciting conductor 83a ispreferably energized through an inductance 86 and condenser 87 tuned tothe frequency of oscillator 88. The second harmonic output is thenobtained from pickup conductor 81 in the same manner as in FIGURE 5a tobe now described. Polarizing flux'may be supplied by means of battery 89and resistor 89'. The oscillator 88 and battery may be connected toconductor 81 if desired, and conductor 83a omitted.

I-t will be observed that the conducting strip 81 forms a portion of asingle turn primary winding 90 .which is coupled to a high impedancesecondary winding 91 on a transformer core 9 2'. by means of a U-strap90a. The transformer construction may be similar to that shown in myPatent 2,585,065. 'Ihe secondary winding 91 of the transformer is tunedby means of a variable capacitance 93 to Ythe second harmonic of theoscillator frequency from 84 in FIGURE 5a (or from oscillator 88 inFIGURE 5b), and the output is delivered to a tuned amplifier 94 anddetector 95 for driving a suitable output device such as aloud speaker96. As previously, if no D.C. polarizing uxis introduced into the core80, the detector 95 must take the form of a phase demodulator forobtaining an output with polarity varying in accordance with therecorded signal on the tape.

FIGURE 6 illustrates a further form of modulator head wherein themagnetic circuit has a gap therein between a pair of pole shoes and 101.The upper edges 100a and 101a have a non-magnetic gap 102 therebetweenand receive a record member 104 thereacross as in the embodiment ofFIGURES 1 4. Leads 105 and 106 connected to series opposing coils 107and 198 are preferably excited with a high frequency signal, while leads111 and 112 are connected with series aiding coils 113 and 114 whichyare utilized to pick up the output signal. Bias is applied by means ofbattery 115, and resistor 116, while the output is taken throughcondenser117, tuned amplifier 118, detector 119 and loudspeaker 120.

The high frequency ux path in FIGURE 6 is partly in air. The signal fluxpath preferably includes saturating strip 121 between the lower ends ofthe core pieces. The thin saturating strip is preferably secured to anonmagnetic backing member 122.

FIGURES 7 and 8 illustrate a head which may also involve high frequencyflux paths through air wherein pole shoes and 131 are mounted on brassbackbone pieces 132 and 133 which may be clamped together with asuitable gap spacer 134 therebetween to define the non-magnetic gap. Thepieces 132 and 133 may be secured, for example, by inserting cement inthe space 136 therebetween. The pieces 132 and 133 are recessed toreceive the coils 140 and 141 which may be arranged and excited asindicated in FIGURES 4 and 6. The saturating strips 143 and 144 may besecured in .low reluctance relation to pole shoes 130 and 131 by meansof plates 146 and 147 which in turn maybe cemented in place. At thelower end, the magnetic circuit may be completed by a pair of yokes 150and 151 which may also be cemented in place on the backbone 132, 133.

By way of example each coil may comprise 20 turns of No. 30 HF woundbiilar in two layers on a 4&2 inch diameter form. The gap spacer may bea .0002 inch is preferably acont-1.9.15

nicwthe 1pole's`hUe's=may?be :006 .'by 3&2 inch*Mumetal umd theannulationl strips :may bel-.001 'by 5&2 by 'J1/' -inch "fMlypermalloy"vFIGURES 9 .and l0 'illustrate a .magnetic modulator dread utilizing asingle satnratng strip v180 completing 'nt magnetic circuit vwi-thipole'shoes 1'81 and `1182 iand non- :magnetic gap 183. The pole fshoes'maybe vsoldered to- 4igether 'as indicated lat :184 vand :the fsaturating:strip 180 may be clamped with turned-n portions 181a and '182e ioftthepole'shoes by :means of'p'lates 186 and 187. An iinner -winding.i190 .is connected to an oscillator =191 by means of leads..192 andA193 "and provides thigh 'frequency rti'elds -duc to the -oppositelywound `portions 190a Vand `19017 thereof as indicated in FIGURE l0. The.signal H p'ickup winding 194 has leads 195 and 196 connected fto.amplifier 197 tuned vto the 'second .harmonic of vthe :oscillatorfrequency, detector 198 which may be a con- *ventional amplitudemodulation rdetector such 'as .found :in ra'dio circuits, poweramplifier v198 'and loudspeaker y199. 'I'he detectors kof FIGURES 5 vand6 aswell as :FIGURES `1`2 to l5 and 17 may, of course, include:power-amplification stages if necessary. Bias flux 'greater than themaximum signal linx Imay be provided by bat- -tery 200. This .headconstruction :has the advantage of .Providing oppositely directed high.frequency fluxes at :the gap v183 while requiring .only -a single:saturating strip :.for .ease 'of assembly. By vway 'of example theinner high frequency winding 190 may comprise 30 turns of No. 33 HF, 15right hand fturns and .15 left hand turns in onelayer and have an.inside `diameter `of g2 'of an inch, while the signal coil ".maycomprise .30 .turns No. 33 IHF all-1in the :same direction intwolayerswith an-inside diameter of 52 of an inch. The oscillator coil may be 9&2of 4Van inch long while Athe 'signal coil may be '%4 of aninch'long. Thesingle .modulatorfstnp 4may :advantageously be .001 by -lz'in'ch .FIGUREll illustrates `a head construction similar :in operation to that'ofFIGURES '9 and 'l0 wherein a signal 'coil 194is `connected in a circuitas shown in .iiIGURE l10 and vis wound around a high frequency coil (notshown) which is wound part in one direction and part .in the oppositedirection as the winding 190 in .FIGURES 9 `and 10 -and is energized asthe winding "190 in FIGURE 10. The rsatura-ting member 201 is of:reduced cross-section as in the lprevious embodiments. .fllhepolc shoes203 and 204 define a non-magnetic gap #205 and are in magnetic circuitwith the saturating strip S201 by means of core portions 207-2l0.

FIGURE .12 illustrates a head .construction utilizing n single 'winding220 all wound in the same direction 'on a fsaturating strip 221 bridgingbetween va pair of pole pieces 222 and 223 defining a non-magnetic gap224. .The 'winding 220 is excited by means of an loscillator .225'through capacitance y226 and inductance 227 'tuned ato the fundamental:frequency of oscillator 225. The :output .is Ptaken 'from 'the windingV220 through an amapliler i229'tuned to twice the frequency of theoscillator 225, a suitable detector 230 and output device 231.Polarizing ilux 'is introduced by means of battery 233 andtresistor'234. PIt will be observed that the saturating istrip 221 may haveenlarged portions 221a engaging flat- Wisewith the lower faces of thepoles 222 and '223 to `4provide .a complete loop magnetic circuit. Thishead construction has the disadvantage that the fundamental *frequencycomponent 'fromthe oscillator is not balanced 'out satrthe gap, -so that-thehigh frequency excitation 'must beadjusted -to a value too .low .tocause erasure of .the rsignal'uon record .member 1235. In practice, witha thin v4saturating strip portion 22lb, .it has been .foundsubstanttially impossible to produce an appreciable erase vfield `atgap224. dtamaybe .noted that in .magnetic -circuitswhere the :xamrating.strips are '.nfseries, the stri-ps maybe .twice csovi'defas where thestrips `ar'e'in parallel,- providing .a

Vof the ux from the record member .passeslthrough each of the saturatingstrips. It `may also be noted that with the embodiment of FIGURES 9 and.10,'there is the advantage that the two winding :portions a and 1901)may be formed from the same electrical conductor thereby avoiding thenecessity for interconnecting `two windings in series or parallel.

While the windings have been described as being energized by alternatingelectrical energy, other suitable excitation energy may be employed withAcorresponding modifications inthe electrical output 'means for derivingan electrical signal from the resultant llux variation in the head.

Referring now to FIGURE 13, a further embodiment 'has been illustratedwherein a magnetized record memlber 250 travels across pole shoes 251and 252 at the nonmagnetic gap 253 therebetween, and the pole shoes havea thin saturating st-rip 256 wit-h enlarged end portions 25611 .inatwise engagement with the inturned ends of 'the pole shoes and areduced cross section .central portion 256b of similar outline to thatshown at 221 in FIGURElZ. The thin saturating strip may -be carried by asuitable nonm-agnetic support 257 of corresponding configuration and maybe secured thereto as Vby a suitable cement .for ease in handling of thesaturating strip. Polarizing ux may `:be introduced by a very weakmagnet 'such fas indicatedaat 259 which in practice may comprise a.magnetized piece of magnetic recording tape less than a 'quarter:inchin width. The required value of polarizing fuxfhas been foundinsuiiicient to affect the tape `250 at the fgap `253.

As previously described, the polarizing tlux in the 'saturatin'g stripis preferably greater than'the 'maximum signal flux produced in thesaturating strip by the record member 250. This same relationship holdstrue for the .embodiments of FIGURES 1 to l2. f It will fbe observed inFIGURE 1.3 'that'the hig'h sfrequency excitation is supplied -from anoscillator i260 through condenser 261 and inductance 262 tuned'to thefundamental frequency of the oscillator 260. The'win'dings on thesaturating strip are bilar wound with one winding portion 264 connectedin series with an inductance 265 and the high frequency 'oscillator260,while'the 'other bilar winding portion 267 is connected 'in serieswith -an inductance 268 and the input terminals of ran lamplifier 269preferably tuned to an even order harmonic 'of-"the fundamental ofoscillator 260. The voutput of thetuned amplifier 269 is fed throughdetector '270 to the output device 271 which may be a loud speaker.'Theinductances 265 and 253 are coupled in such a way as to balance'o'utthe fundamental component induced-in the output `wind- 'in'g 267, sothat with no signal introduced at the 'gap 253, there will be nofundamental frequency component in'the input circuit of tuned amplifier.269 including coils 267 and 268. It will be understood that as asgnal"tluxfiis introduced into the saturating strip 256, the "fundamentalcomponent in the input circuit to tuned amplier "269will be unbalancedand there will be a fundamental `frequency perniIc having a compositionof 50% .nickel and the .remainder iron and minor constitutents, orPerminvar having a composition of 25 cobalt, 45% the embodiment, the

remainder iron and minor constituents. Further, in this high frequencywindings 304 and 305 are preferably connected in series aiding relationwith respect to the gap 301 so as to actually excite the magneticmaterial of the record member 302. The high frequency magnetic intensityat the gap 301 is preferably of a small amplitude in comparison with thecoercive force of the record member. With such excitation, the magneticmaterial of the record member operates on a minor hysteresis loop, theincremental permeability of which depends on the residual magnetizationof the portion of the record member at the gap 301.

In this embodiment it will be apparent that the head responds not to theexternal leakage ux from the record member, but to the actual internalmagnetization of the record member. The signal flux acting on t-he headis thus independent of the recorded wave length of the signal so that afundamental defect of heads relying on leakage ux from a record memberis overcome. Signal pickup windings 310 and 311 may be bitilar woundwith windings 304 and 305 respectively, and also in series aidingrelation with respect to the gap 301. The oscillator circuit is providedwith an inductance 312 and the pickup circuit is provided with aninductance 313 coupled therewith, so that the coupling betweeninductances 312 and 313 may be varied to balance out the fundamentalcomponent in the pickup circuit in the absence of a signal flux from thetape 302. Polarizng ux may be introduced by means of a magnet 315. Anamplifier 316 has its input connect- -ed to the pickup circuit and ispreferably tuned to an even 1 :order harmonic of the frequency of theoscillator 317.

The output of the tuned amplifier 316 lis connected through aconventional amplitude modulation detector and power amplifier unit 318to an output device 319 such as a loud speaker.

In FIGURE 15, a magnetic head is illustrated having a pair of pole shoes320 and 321 defining a non-magnetic gap 322 receiving a record member323 thereacross. A relatively thin flat member 325 of magnetic materialbridges across the pole shoes 320 and 321 and is disposed in 'llatwiseengagement with the ends of the pole shoes 320 and 321. The member 325may have a window 325a therein to denne a pair of thin saturating strips325b and 325e. A magnet 327 is illustrated as applying polarizing fluxto the strips 325b and 325e, the magnet being too weak to affect therecord member 323. In this embodiment, windings 330 and 331 are excitedby means of fluctuating electrical energy in the form of a lseries ofunidirectional rectangular pulses such as indicated schematically at 334from a pulse generator 335. The windings 330 and 331 are preferablyconnected in series opposing relation with respect to the gap 32.2 sothat there will be no net exciting iiux at the gap. Pickup winding 338and 339 may be connected in aiding relation with respect to the gap 322and be connected through an integrating circuit such as resistance 340and capacitance 341 to a detector 342 and output device such as a meter343. It will be understood that pulse operation of the head makespossible multiplex operation wherein a multiplicity of channels may besuccessively scanned by means of successive pulses to successive headsso as to reproduce a signal distributed across a multiplicity ofchannels. In this instance, each head would be pulsed at intervals asshown in FIGURE 1S. In the illustrated embodiment, the output at 343would consist of a series of pulses at the frequency of pulses 334 andvarying in amplitude and polarity in accordance with the signal on .therecord member 323.

FIGURES 16 and 17 illustrate one manner in which .a saturating strip 360may be excited without winding a conductor continuously therearound in ahelix as illustrated in the preceding embodiments. In this case, as

illustrated in FIGURE 16, a conductor 36'1 is wound in one or morelayers as illustrated with successive parallel .portions 361:1, 361b,361C, 361d and 361e. The winding` aiding relation with respect to gap383,

361 is then' folded oula 'median plane 'indicated by the dot-dashline3463, and the saturating strip 360 slipped into the folded windingas illustrated in FIGURE 17. 'It will be observed in this embodimentthat the conductor portions 361a, 361e yand 361e on each side of thestrip 360 will induce tiux in one direction longitudinally of the strip360, while the conductor portions 361b and 361d will induce tiuxes inthe opposite direction on both sides of the strip.

FIGURES 18 and 19 illustrate a Afurther way in which a saturating strip360 may be excited. Here, a conductor 366 is wound in a sinuous mannerwith successive parallel portions 366a, 366b, 366e, 366d, 366e and 366f.:The winding is then folded on a medi-an line 368 to receive thesaturalting strip 360 as indicated in FIGURE 19. In this embodiment, itwill be observed that the conductor portions 366a, 366e and 366eoneachside of the strip 360 induce magnetic fluxes in one direction whilethe alternate conductor portions 366b, 366d and 366f induce magnetictluxes'in the opposite direction.

It will be understood that in both FIGURES -17 and 19, the pickupwindings can be wound on pole `shoes such as 380 and 381 in FIGURE 17.Specifically, wind$ ings 382a and 382b .may be provided connected inseries the output being fed to a timed amplifier and detector unit 384and then to a loud speaker 385 as in previous embodiments. In this case,the head may also be conveniently utilized as a recording head, thewindings 382a and 382b being connected with a suitable input 388 bymeans of a switch 389 to record on'an unmagnetized vrecord member 390.It will be understood that the poleshoes 380 and 3&1 must be ofsuticientcross-section to carry the required recording ux.

As previously, the saturating strip 360 is preferably of greatly reducedcross-section in relation to the crosssection of the pole shoes 380 and381. The totaler-osssection of the saturating strip 360 is required tobe a fraction of the cross-section of the record-playback pole pieces380 and 381. It may be noted that placing the windings 382a and b on thepole shoes rather than on the saturating strip results in an appreciablereduction in output voltage; however, with suitable operatingconditions, the output is still superior to that obtained with that ofinduction type heads responding to the rate of change of signal linx.The record-playback head of FIGURE 17 is responsive directly to thesignal flux at the gap 383 to give greatly improved low frequencyresponse characteristics to the head and thus avoiding the need for -lowfrequency equalization as required with an indue tion type head.Further, the output level is such -as to avoid the need Afor a high gainamplifier which tends to introduce hum and other noise into the output.

FIGURE 20 illustrates the relation of output to the applied intensity ofthe magnetic eld established in the saturating strips. This relationshipis applicable to the embodiments of FIGURES 1 to '13 and 17. Curves 400and 401 illustrate the signal and noise output in decibels (on differentscales) while curve 403 is a plot of signal minus noise in decibles as afunction of the intensity of the magnetic tield established by theoscillator in the saturating strip. Point 404 on curve 403 maycorrespond to a signal minus noise value of 68 decibels. Point 405 maycorrespond to a signal minus noise value of 76 decibels. 'I'he verticalline 408 may correspond to the intensity of the magnetic field in thesaturating strip just producing the saturation value of intrinsicinductionin the strip. It will thus be observed that optimumsignal-tonoise ratio is obtained by driving the oscillator to producevalues of magnetic intensity inthe region of or preferably above thesaturation value of magnetic intensity for the saturation strips. Itwill be understood from FIG- URE 20 that the maximum value of H due tothe oscillator excitation is preferably greater than the maximum. I-Iprovided by the polarizing source and the signal maglagoon-'frais 'ofthese embodiments must be less than about where B, is the maximumresidual induction ofthe .record member, A, is the cross-sectional areaof the magnetized portion of the record member, P is the ypolariri-ngflux in the saturating strip, F is any feed back flux which may beintroduced into the saturating strip, and B,l is'the intrinsicsaturation induction for the saturatng strip, the valves being taken inconsistent units. This limit of about 50(B,Ati-P+F) .Bl is theapproximate maximum limit for -usefulsreproduction of music sincemusical reproduction with cross sections appreciably above this limitproduces a-result which is usually unacceptable to the listener .and tothis extent -represents the critical limit for usefulreproduction.

Feedback is illustrated .in FIGURE 4 as being taken at the output of astage of amplification indicated at 420 and prior to a further stage ofamplification '420" by means of a capacitance 421 and resistance 422coupled to signal windings 29b and 30b by .leads 43 and-'44. With onepolarity of the battery 50, the feedback will -be negative, while withthe other polarity ofthe battery 50, the feedback will be positive. Itwill be observed from the above formula that with positive feedback, andfor a given signal-tonoise ratio, the maximum permissable area of thesaturating strips is increased. while withnegative feedback, the maximumpermissable cross section is reduced. It -has been found that the use ofnegative or inverse feedback is an elective way of improving thelinearity, reliability, and stability 'of vthe heads of the presentinvention. With a sutlciently high --feedback factor, all distortion andgain variations, except those in the tape recording itself and effectsdue to varying degree of contact between the tape and playback head, canbe reduced to negligibly small eiects. It was Ifound that with theproper value of the feedback components, the net output signal from aconstant level recording on tape was essentially unchanged in spite ofartificially produced large changes in the gain of the amplifier.

For each of the embodiments of FIGURES 1 to 13, 1S and 17, thepolarizing iiux is preferably of magnitude greater than the maximumsignal ux in the saturating strips.

For a useful signal-to-noise ratio for reproduction of music, it hasbeen found that the Iarea of the saturating strips must be less than l5nimes the cross-sectional area of the magnetizable portion of the recordtape where a polarizing flux is present and tape has a maximum uxcapacity of approximately 1 maxwell. Ihis limit is reduced by a Ifactorof two where there is no polarizing ilux and is reduced proportionatelyif the residual flux capacity of the tape is less than one maxwell, orincreased proportionately if the flux capacity is greater than onemaxwell. For example, for a llux capacity of .5 maxwell, the limit wouldbe 71A times the cross-sectional area of the magnetizable portion of thetape.

It has been Ifound that for fine scanning of the record member, a smallgap is required. This in turn tends to reduce the amount of tlux linkingthe head and to require a relatively small saturating strip. Withrelative Vof the saturating strips.

ly small signal 4ux linking'the Asaturating strip, it'is advantageous touse a high frequency excitation in'thesaturating strip. However, corelosses `are not prohibitive at the high frequencies because ofthe Vsmalldimensions 'I'he vpresent invention vthus makes Ifeasible a very tinescanning gap while providing the required high output level for a goodsignaleto-no'ise ratio.

YIt may be noted that negative feedback reduces the eiective signal lluxin the saturating strips, yand t-.hus allows for a further reduction inthe minimum size of the strips for distortionless output. On the otherhand, 4"a positive `feedback increases the permissible size of thestrips by eEectively adding to the signal flux.

The following modifications in the illustrative embodiments maybe notedby way of example. In "FIG- URE 5a, a single loop conductor may excitethe 'core rather than a multiplicity of turns. Further, a conductorcarrying current of one phase may extend 'on one side of the core, and aconductor carrying currcnt'of the opposite phasemay extend on the otherside of the `core to excite the core with bucking high frequencyexcitation with 'respect to the gap without actually wrapping aconductor completely around 'the lower saturating strip portions. Itwill be understood that the windings and 141 "of FIGURE 7 may be excitedin vt-he manner illustrated in FIGURE 4, and the 'output taken vby means"of the circuit'showu in VFIGURE 4. In FIGURE 6, 'the saturating strip1211 may be omitted to leave a ga'p `across the lower portion of thecore, and "the 4head 'will operate, although unless the legs 'of thepole 'pieces' 100 and 101 are of relatively smallcross-sectiomtherestiltin'g noise level will make the head unusable forthe 'playback of music signals. The oscillator coils are 'preferablylinked to the pole legs at .regions relativelyremote from the gap, so asrto avoid the' possibility of erasing eifects upon the tape due'to thehigh frequency excitation ux generated by the coils.

It may be noted that in the embodiments of FIGURES 1 through 14 and 17,the oscillator 'excitation may be tuned to a frequency of 500 kilocyclesper second, -for example, and the output of t-he pickup coil may bedelivered to a conventional radio set tuned to l megacycle per second.In such a case, polarizing iiux is most conveniently supplied by meansof a permanent magnet arrangement such as illustrated in FIGURE 13 or14. With one arrangement utilizing thin cross section saturating 'stripssuc-h `as disclosed herein it'was found that the noise from the head wasdefinitely below that of (the tape itself. It was also Ifound thatoptimum performance in the playback system corresponded to aconsiderably higher oscillator input to the head than would be expected,and in fact some heating of the head occurred at the optimum value ofoscillator input. However, the required power is obtained from theminiature 12AU7 oscillator tube without ditliculty and no erasure of thet-ape 'was found.

It will be understood that the heads illustrated in the drawings may beused -for recording transversely of the tape, for example, with the gapat an angle other than a angle to the path of travel of the tape or evenparallel to the path of travel of the tape.

Further, the pole pieces may be offset parallel to the long dimensiono-f the gap to provide closely abutting parallel gap surfaces whichoverlap for only a fraction of the total extent of the respective gapsurfaces. For example, with gap surfaces 425 and 426 offset laterally ofthe tape 427 as shown in FIGURE 21, and extending at right angles to thedirection of travel of the tape as indicated by arrow 428, a shortwavelength recorded signal will be concentrated in a longitudinallyrecorded eld at the central portion of the tape which travels across thelongitudinally spaced overlapping portions 425a and 426a of the gapsurfaces of the poles. However, for long wavelength recording, diagonalmagnetiza- `of the organ.

tion'of the outer portions of the tape 427 will occur for examplebetween the exposed gap surface 426b of one pole and a side surface 425eof the opposite pole, which side surface may, for examp1e, extendparallel to the direction of the tape travel. 425a and 426a may have alength between zero and several gap widths. This olf-set shoeconstruction in the head results in response to infinitely longwavelengths as well as good short wavelength resolution.

As indicated in FIGURE 5a, a pair of core pieces 500, 501 of magneticmaterial may be glued, cemented or otherwise bonded -to the poleportions of the nI lamination so as to enlarge the tape contacting areaof the head and improve the long wavelength characteristics thereof. Theadditional core pieces may be made of a suitable ferrite material whichmay be a homogeneous crystalline material composed of ferrie-oxide andthe oxide of another metal.

The embodiment of FIGURE 15 may be utilized in an electronic organ. Inthis application the pulsed head acts as a frequency changing devicewhich reproduces the waveform of a recorded repeating signal at anydesired frequency. With this system, there is required for each pitch ofthe organ a pulse generator, and a magnetic drum with recorded wave formis required for each stop Thus, an organ with 73 pitches and 20 stopswould have 73 pulse generators and 20 magnetic drums, a total of 93basic elements replacing the 1460 pipes of an equivalent pipe organ.

With respect to the embodiment of FIGURE 13, the modulator strip 256 iscut to shape with the wide end portions annealed at, and then cementedto a Bakelite stiiening piece such as indicated at 257. The coils arethen wound on the strip assembly. With this construction procedure thereis a minimum possibility of straining or otherwise damaging themodulator strips.

It will be apparent that many further modifications and variations maybe effected -without departing from 4the scope of the novel concepts ofthe present invention.

I claim as my invention:

l. A magnetic transducer device comprising a magnetic core providing asingle magnetic path, the Core being adapted to receive flux from anexternal source linking said path, means including winding means forgenerating opposed exciting fluxes in said magnetic path, a strip ofreduced cross-section forming at least a portion of said magnetic path,and said winding means being provided by a single conductor havingportions opposite- The overlapping portions `ly wound onsaid strip, andsignal winding means linkv`ing the portion of said strip on which saidconductor is wound in radially oEset relation.

A 2. Magnetic apparatus comprising magnetic core means providing a loopmagnetic ilux path, means for introducing a signal ux into said looppath, said path including an elongated stnp of magnetic material ofrelatively small cross section in comparison to the cross section ofother portions of said core means, means comprising exciting windings onsaid strip for producing opposed exciting uxes in said strip, and outputmeans on said strip and` separate from said exciting windings andcoupled to said exciting fluxes for producing an electrical output.

3. Magnetic apparatus comprising magnetic core means providinga loopmagnetic ux path, means in series in said loop magnetic flux path forintroducing a signal ux into said loop path, said path including anelongated strip of magnetic material of relatively small cross sectionin comparison to the cross section of other portions of said core means,means comprising exciting windings on said strip for producing opposedexciting fluxes in said strip, and means coupled to said exciting fluxes-for producing an electrical output, said last-mentioned meanscomprising a pickup winding on said strip.

v4. Magnetic apparatus comprising a magnetic vcore providing a seriesmagnetic path and including means for introducing a signal ux in saidpath, said core including a portion which saturates at a lower value ofiiux than other portions of said core, exciting Winding means on saidportion for generating oppositely directed exciting fluxes at sectionsof said portion offset along said path, and an output winding encirclingsaid portion in overlapping relation to said exciting means.

References Cited in the le of this patent UNITED STATES PATENTS2,608,621 Peterson Aug. 26, 1952 2,700,703 Nordyke Ian. 25, 19552,722,569 Loper Nov. 1, 1955 2,804,506 Schurch et al. Aug. 27, 1957 iFOREIGN PATENTS 270,675 Switzerland Dec. l, 1950 OTHER REFERENCES EDVACProgress Report #2, June 30, 1946, pages -4-23, py0-l64 and py-0-l65.

